When implementing transmission networks, a synchronizing rate must be provided for certain equipment making up these networks. This is particularly the case for UMTS (Universal Mobile Telecommunications System) transmitting/receiving base stations, referred to as Node B, and, more generally, for all equipment supporting real-time applications or services.
At the present time, this synchronizing rate is delivered by a synchronizing signal originating from a primary clock exhibiting an accuracy, with respect to the UTC (coordinated universal time) frequency, of 10−11. This synchronizing signal is transported over the various parts of the network, particularly in the core, collecting or access parts, via the SDH (Synchronous Digital Hierarchy) transport layer or possibly the PDH (Plesiochronous Digital Hierarchy) transport layer.
The gradual but regular introduction of Ethernet technology for implementing networks or parts of networks has now created the problem of transporting and preserving this synchronizing rate on such networks or parts of networks.
The abovementioned problem is likely to act as a brake on the deployment of this type of network, in particular in their application to the UMTS collecting network.
Until now, there have essentially been two types of solutions proposed for transmitting and preserving the synchronizing rate:
a method referred to as adaptive, which involves retrieving the transmission rate at the network input from the rate of arrival of data packets into a receive FIFO memory, the read rate frequency of which is dependent on the level of filling of this memory;
a method referred to as a time stamp method, which involves transferring in the band, i.e. in the frames, in the form of three or four bytes, information relating to the transmission clock, in order to be able to reconstruct, at the receive end, the transmission clock rate.
In the case of the two abovementioned types, or even in the case of a combination thereof, the reconstructed or restored synchronizing signal or rate is highly dependent on the network topology, in particular on the number of nodes crossed, as well as on variations in traffic load on this network. The above-mentioned dependence results, overall, in a variation in the time for transferring packets, a phenomenon referred to as latency, when the latter pass through the network from end to end, thereby producing variations or shifts in phase, which are passed on to the reconstructed rate.
Consequently, the reconstructed rate is very poor in quality as regards synchronization and may turn out to be non-compliant or at the limit of compliance with the currently applicable standards or recommendations.